With high-power lasers, there has long been the desire for the TEM OO mode. The mirrors play a part in the possible distortion of the beam. The surface of the mirrors deforms, inter alia, due to the fact that they heat up. It is true that the mirrors are cooled. Nevertheless, the cooling cannot be ideal. In addition, if a multiplicity of mirrors are used, the cooling may differ, so that not only is there deformation of the mirrors but different deformation. The constructural measures to separate the cooling fluid from the laser gas are enormous.